I know how Adobe Digital Editions (the programme) works.
But there is a difference between Adobe Digital Edition (the format - which doesn't work on the Nook) and Adobe ePub.
And S&S offers Zero Sum Game only as an Adobe Digital Edition but not as an epub so far as I've seen.
Here I believe you to be incorrect. The nook does read adobe epubs that are authorized by Adobe Digital Edition. When the nook is connected to your USB port and ADE is running you can authorize the nook as a recognizable device and any adobe epub or pdf that has DRM can then be sideloaded to the nook and read. I have done it a dozen times with no problems. This is one of things I like about the nook is not being locked into one distributer of ebooks.
Kevin